Field of the Invention
The present invention relates to a method and system for cleaning a substrate, such as a semiconductor wafer, a substrate for a liquid crystal display (LCD), a glass substrate for use in manufacture of a plasma display panel (PDP) or the like as well as a method and apparatus for making a sherbet-like cleaning composition for cleaning of the substrate.
In Japanese Patent Laid Open No. 11-151467 in the name of the assignee, a method and system for cleaning a substrate by scrubbing has been proposed.
The proposed system comprises an apparatus for producing snow ice crystal grains of a predetermined size, holder means for holding a substrate to be cleaned, supply means for separately supplying to the substrate the snow ice grains for mechanical cleaning of the substrate and a chemical agent for chemical cleaning of the substrate to form a sherbet-like aggregate over each of the opposite surfaces of the substrate, means for urging the sherbet-like aggregate against the opposite surfaces of the substrate and means for moving the aggregate relative to the substrate surfaces. The snow ice producing apparatus is a cyclone type one and is constructed in such a manner that the ice crystals are created by injecting cooling air into the cyclone chamber, as is well-known to those skilled in the art.
In view of the fact that the chemical cleaning of the substrate surfaces may be accomplished by the chemical agent and also the mechanical cleaning thereof can be effected by the scrubbing action of the snow ice grains during the movement of the sherbet-like aggregate relative to the substrate while urging the aggregate against the substrate surfaces, the proposed cleaning system can provide effective and efficient cleaning of the substrate without causing any appreciable damage thereto. In the following description, this process will be referred to as a xe2x80x9cscrubbing cleaning processxe2x80x9d.
However, the conventional scrubbing cleaning process as described above has the following drawbacks:
Firstly, after the semiconductor wafer is subject to a polishing process, in particular, chemical-mechanical polishing (CMP) and then often exposed to pure water, numerous contaminant particles can be found on the polished wafer surfaces. Typically, these particles are adhered to the wafer surfaces mainly due to the effect of the surface tension force of the pure water remaining thereon. A greater scrubbing or shearing force is required in order to remove the particles from the wafer surfaces to overcome the surface tension force of the pure water. On the other hand, the increased scrubbing force resulting from the movement of the sherbet-like aggregate relative to the substrate can adversely affect the polished wafer surfaces, depending on the type of the substrate and the amount of the scrubbing force applied on the substrate surfaces.
Secondly, it is difficult to provide the aggregate of the snow ice grains and the chemical agent with the desired characteristics, for example, a suitable consistency. Also, it is necessary to control the size of the snow ice grains in accordance with the type of the substrate to enable efficient scrubbing and cleaning of the substrate. More specifically, in view of the fact that microscopic surface asperities can vary in size in accordance with the type of the substrate, it is important that the snow ice grains have a size substantially smaller than that of the depressions of the substrate surfaces in order to enable to be removed or scrapped dust particles entrapped in the pits of the substrate surfaces. As can be appreciated by those skilled in the art, oversized snow ice grains may fail to remove the dust particles from the substrate surfaces.
The scrubbing cleaning process utilizes an on-site mixing at the substrate surfaces where the properly sized snow ice grains and the chemical agent are mixed to produce a sherbet-like material for chemical and mechanical cleaning of the wafer. Such an on-site mixing may not provide enough control to obtain sherbet having the desired nature or consistency than an off-site mixing process. Also, it is somewhat difficult to control the size of the snow ice grains after the on-site mixing. Further, the on-site mixing process may prevent effective production of the sherbet-like material.
Thirdly, in the conventional snow ice making apparatus, a heat exchange between the spray of the pure water and the cooling gas is brought about by a heat conduction, i.e., via a direct contact of this spray with the cooling gas within the cyclone chamber. The effectiveness of heat exchange depends on various factors, for example, the time during which the pure water spray is in contact with the cooling gas and the contact area therebetween. As a practical matter, successful control of these factors is difficult. Further, the ice snow grains tend to adhere to the inner wall of the cyclone chamber to grow into a block or column of ice thereon, which black or column of ice may prevent the heat exchange between the water spray and the cooling gas as it creates an impedance to the rotational movement thereof within the cyclone chamber, thereby make it difficult to establish a suitable contact timelength and a contact area amount, which in turn may also contribute to reducing heat exchange effectiveness. Such a process and the on-site mixing process can result in inefficient production of the sherbet-like material.
Supercooling of the pure water in a mixing vessel to below its freezing point by use of an external cooling means does not contribute to improving the effectiveness of making the sherbet-like material because the ice crystals tend to form simultaneously on the cooled inner wall of the chamber which the pure water comes into contact with, to grow into a block of ice all over this inner wall overtime rather than to grow into ice grains.
It is therefore an object of the present invention to provide a new method and apparatus for making a sherbet-like cleaning composition having the characteristics suitable for the type of the substrate to be scrubbed and cleaned.
It is another object of the present invention to provide a new method and system for cleaning a substrate in an efficient and effective manner using the sherbet-like cleaning composition while avoiding the limitations of the conventional method and system as described above.
With these above objects in mind, the present invention provides a method for cleaning a substrate, comprising the steps of providing a sherbet-like cleaning composition consisting of snow ice crystal grains and a liquid organic chemical agent and moving the sherbet-like cleaning composition relative to the substrate to be cleaned along at least one of the surfaces thereof, the organic chemical agent of the composition operating to reduce the adhesion of residual particles to the at least one surface of the substrate, the sherbet-like cleaning composition having a consistency allowing the particles on the at least one surface of the substrate to be entrained therein to thereby enable the particles to be removed from the at least one surface of the substrate.
Preferably, the step of providing the sherbet-like composition includes the steps of mixing the liquid organic agent and pure water in a predetermined mixing ratio in a mixing vessel to form a mixture, and supercooling the mixture uniformly therethroughout at a predetermined temperature lower than the freezing point of the pure water and greater than that of the organic chemical agent while stirring the mixture in a manner permitting the pure water to change into snow ice crystal grains.
Preferably, the stirring step includes the steps of creating vortices in the mixture, growing the vortices and diffusing the grown vortices in the mixture.
Preferably, the step of providing the sherbet-like composition can further comprise the step of controlling the mixing ratio and/or the degree of supercooling of the water that produce the snow ice particles having a predetermined ratio by volume in the pure water, and thus the sherbet-like composition produced has a predetermined consistency.
Further, the step of providing the sherbet-like composition further comprises the step of detecting a stirring torque applied on the mixture in the mixing vessel and controlling the degree of supercooling of the mixture in accordance with the detected torque in a manner so that the sherbet-like composition produced has of a predetermined consistency.
The present invention also provides a method for cleaning a substrate by means of a sherbet-like composition containing snow ice grains and a liquid chemical agent, comprising the steps of providing the sherbet by mixing the liquid chemical agent and pure water in a predetermined mixing ratio to form a mixture and then supercooling the mixture uniformly therethroughout while stirring the mixture at a predetermined temperature lower than the freezing point of the pure water and greater than that of the chemical agent to produce a sherbet-like composition having the characteristics suitable for scrubbing and cleaning the substrate, and urging the sherbet against at least one of the surfaces of the substrate while effecting relative movement between the sherbet-like composition and the substrate to scrub the at least one of the surfaces of the substrate to clean it.
Preferably, the step of providing the sherbet-like composition can further comprise the step of controlling the mixing ratio and/or the degree of supercooling of the mixture so that the snow ice grains contained in the sherbet-like composition have a controlled size.
Preferably, the stirring step further includes the steps of creating vortices in the mixture, growing the vortices and diffusing the grown vortices in the mixture.
The chemical agent may include organic chemicals, alkaline chemicals, acid chemicals or any combination thereof.
Preferably, during cleaning of the substrate, the substrate can be rotated and reciprocated.
The present invention also provides a system for cleaning a substrate, comprising an apparatus for producing a sherbet-like composition consisting of snow ice grains and a chemical agent, holder means for holding a substrate to be cleaned, supply means for supplying to the substrate the sherbet-like composition for cleaning of the substrate, and means for moving the sherbet-like composition being supplied relative to the at least one of the surfaces of the substrate, the apparatus for producing the sherbet-like composition comprising a mixing vessel for mixing the chemical agent and pure water in a predetermined mixing ratio to form a mixture, means for supercooling the mixture uniformly therethroughout at a predetermined temperature lower than the freezing point of the pure water and greater than that of the chemical agent, an impeller blade having a generally vertically extending axis of rotation for stirring the mixture uniformly within the mixing vessel to produce a sherbet-like composition, the impeller blade having side edges which are each located so that there is sufficient gap between each edge and the inner wall of the mixing vessel to prevent the edges from coming into contact with the inner wall of the mixing vessel, during the rotation of the impeller blade, the sherbet-like composition being used to remove particles from the at least one of the surfaces of the substrate.
Preferably, the impeller blade comprises a contoured plate member having first and second side edges located opposite from the axis of rotation, the first side edge extending in a direction parallel to the vertical axis of rotation of the impeller and the second side edge being formed to have a sawtooth waveform shape. The impeller blade has passage means extending in a direction parallel to the vertical axis of rotation and located adjacent to the first side edge for passing therethrough the materials being mixed.
Preferably, the passage means comprises openings aligned with each other in a direction parallel to the vertical axis of rotation of the impeller blade, and the second side edge has alternate raised and recessed portions, the raised portions being aligned vertically with the corresponding openings and the recessed portions each being located generally at a level between each of the openings.
The present invention also provides a method for producing a sherbet-like composition for scrubbing and cleaning a substrate, comprising the steps of mixing a liquid chemical agent and pure water in a predetermined mixing ratio to form a mixture, and then supercooling the mixture uniformly therethroughout at a predetermined temperature lower than the freezing point of the water and greater than that of the chemical agent while stirring the mixture so that the water changes into snow ice grains a sherbet-like composition including the snow ice grains and the liquid chemicals.
Preferably, the step of providing the sherbet-like composition can further comprise the step of controlling the mixing ratio and/or the degree of supercooling of the mixture so that produce the snow ice particles produced have a predetermined size, and thus the sherbet-like composition produced has a predetermined consistency.
Preferably, the size of each of the snow ice particles is less than 200 micron.
Preferably, the stirring step further includes the steps of creating vortices in the mixture, growing the vortices and diffusing the grown vortices in the mixture.
The chemicals may include organic chemicals, alkaline chemicals, acid chemicals or any combination thereof.
The present invention also provides an apparatus for producing a sherbet-like composition, comprising a mixing vessel for mixing chemicals and pure water in a predetermined mixing ratio to form a mixture, means for supercooling the mixture uniformly at a predetermined temperature lower than the freezing point of the pure water and greater than that of the chemicals in a manner so as to form snow ice grains in the mixture, an impeller blade having a generally vertical axis of rotation for stirring the mixture uniformly within the mixing vessel to produce a sherbet-like composition having characteristics suitable for cleaning the substrate, the impeller blade having side edges which are each located so that there is a gap between each edge and the inner wall of the mixing sufficient to prevent the edges from coming into contact with the inner wall of the mixing vessel during the rotation of the impeller blade.
Preferably, the impeller blade comprises a contoured plate member having first and second side edges located opposite from the axis of rotation, the first side edge extending in a direction parallel to the vertical axis of rotation of the impeller and the second side edge being formed to have a sawtooth waveform shape. The impeller blade has passage means extending in a direction parallel to the vertical axis of rotation and located adjacent to the first side edge for passing therethrough the materials being mixed.
Preferably, the passage means comprises openings aligned with each other in a direction parallel to the vertical axis of rotation of the impeller blade, and the second side edge has alternate raised and recessed portions, the raised portions being aligned horizontally with the corresponding openings and located so that there is a gap between each of the raised portions of the second side edge and the inner wall of the mixing vessel, and the recessed portions each being located generally at a level between each of the corresponding openings.
Preferably, the gap between the first edge and the inner wall of the mixing vessel is less than the gap between each of the raised portions of the second edge and the inner wall of the mixing vessel.
The method according to the present invention further comprises the step of detecting a stirring torque in the mixture within the mixing vessel and controlling the level of supercooling of the mixture in accordance with the detected torque in a manner so as to vary the characteristics of the sherbet-like composition as desired.
According to the first-mentioned substrate cleaning method of the present invention, during the relative movement between the sherbet-like composition and the substrate, the liquid on the substrate surface (for example, the pure water on the substrate surface after CMP) that tends to attract dust particles thereonto can be replaced with liquid organic chemicals. Thus resulting in reduction of the adhesion force between the particles and the substrate surface. In this case, it is possible to clean the substrate surface in an easy and efficient manner by urging the sherbet-like composition having the desired consistency against the substrate surface to cause it to flow over the substrate in such a manner that the particles can become entrapped in or attached to the sherbet-like composition, thereby enabling the particles to be removed from the surface. In contrast to the conventional scrubbing cleaning process, the above-described xe2x80x9centrappingxe2x80x9d process relies largely on the unique characteristics of the sherbet-like composition, i.e., the chemical action of the organic liquid chemical agent as well as the controlled consistency of the agent. Accordingly, any adverse effect on the substrate surface can be avoided.
The substrate cleaning method according to a second embodiment of the present invention relies on the fact that the freezing point of the chemical agent is lower than that of the pure water (xe2x80x9cdepression of the freezing pointxe2x80x9d theory). More specifically, according to the above invention, the pure water can be supercooled while a formation of bulk ice can be prevented by agitating and supercooling the mixture consisting of the pure water and the chemical agents so as to solidify the pure water while at the same time to prevent the chemical agents from being solidified. This enables the efficiency of producing a sherbet-like composition with a desired hardness consisting of the snow ice particles having a predetermined size and the chemical agents in a liquid phase to be enhanced, as compared with a mere cooling process described above. Further, by urging such a sherbet-like composition against the surface of the substrate, the substrate can be effectively cleaned. Control of the mixing ratio and the level of supercooling of the mixture can result in the production of a sherbet-like composition which is in a gel form and contains the snow ice particles having a predetermined size.
In accordance with the teachings of the present invention, the cleaning system of the present invention can carry out either the conventional scrubbing cleaning process or the xe2x80x9centrappingxe2x80x9d cleaning process of the present invention, depending on the type of the substrate to be cleaned. The cleaning system according to the present invention has the advantage that it can produce a sherbet-like composition which can appropriately clean a particular substrate without any modification and additions having to be made, although it is necessary to clean the system using the pure water as cleaning water. More particularly, rotation of the impeller within the mixing vessel while the cleaning water is supplied thereinto will enable the mixing vessel and the impeller to be cleaned. Then, the cleaning water can be passed through the substrate cleaning unit, thereby making it possible to clean the cleaning unit in an easy and time saving manner.